In my experience the UTF-8 support in mIRC is developed enough and fixes more problems than it causes. The only "problem" really is that some luddites complain because they're too lazy to update their IRC clients. ;-) (However, I think that enough time has passed - it's been over a year since mIRC itself started supporting displaying UTF-8 - surely everybody's updated by now.)
I frequently chat in truly multilingual channels and it's very time consuming to instruct newcomers on how to make it possible to enter any character in mIRC. Most of the people don't even realize their characters aren't showing up correctly at the other end until they're told about it.
Therefore, I humbly propose that multibyte display and multibyte editbox are made the default settings along with UTF-8 set to "display and encode" for every new channel and query window.